I am curious about one thing. Most if not all cameras designed specifically for astrophotography use ccd rather than cmos sensors and are cooled by at least 30 degrees centigrade below ambient temperature. Are here inherent advantages of ccd over cmos for this application? Ken

I take your point about the circuitry. I guess they are also simpler because many, and certainly the most expensive shoot in monochrome relying on the use of filters and multiple exposures to capture the RGB channels. That said when you think they have few of the functions of a camera body why are they so expensive?

Here in the UK we are pretty much limited to exposures of no more than a couple of minutes because any more and you are likely to pick up satellite and aircraft trails. Registax has made a great difference to astrophotography. I preface my next question with a very strong warning DON'T DO THIS UNLESS YOU ARE USING SPECIALIST EQUIPMENT AND YOU REALLY KNOW WHAT YOU ARE DOING. If you are photographing the sun using a Hydrogen Alpha filter, would you get better results with a modified DSLR with the IR filter removed?Ken

I presume CCD based cameras and sensors are expensive because they aren't made by the million on a production line like DSLR CMOS sensors are.

If you're using a H Alpha filter on a telescope, that's a very narrow band tuneable filter. I guess your exposure time would be shorter with either the low pass filter removed from the sensor, or with a 60Da vs. a 60D since both will have a stronger response to the H Alpha wavelength. However, if your exposure with the low pass filter installed is short enough not to be a problem, then removing it won't help.

What would help would be a monochromatic camera without a Bayer matrix (or a low pass filter). With a conventional Bayer matrix camera only 25% of the pixels are red, so (unless there is some red response in the green and blue pixels, which there usually is) you only use 25% of the sensor data for H alpha.

Note for those unfamiliar with H Alpha. When an electron makes a particular transition in a hydrogen atom it emits light at a wavelength of 656.28 nm (deep red). Hydrogen is the most abundant element in the universe and is mostly what the sun is made of. Looking at the sun in H Alpha light can show incredible detail in the chromosphere, but it requires a very special (and expensive) filter.